The race has moved swiftly since Iowa and New Hampshire, with national polls now showing Sen. Bernie Sanders, I-Vt., in the lead and Mike Bloomberg, the billionaire former mayor of New York, rising at the expense of others who have been fighting to become a center-left alternative to Sanders.
Everything that was on display Wednesday night - as well as everyone on the stage - showed the tension that now exists and the sense of urgency that time is running out on some of them. Sanders' rise has raised fears that, if he were the nominee, his brand of democratic socialism could doom the party to defeat against President Donald Trump, along with many candidates for House and Senate.
One measure of how rapidly things are changing is this: In barely a week, the question has shifted from whether Sanders has a ceiling, based on the fact that he managed just a quarter of the vote in both Iowa and New Hampshire, to whether he can be stopped. The answer to that question could be known as early as Super Tuesday, less than two weeks away.
Some Democrats see Bloomberg as the one candidate who could sell better in a general election, and there have been suggestions that the time is coming for the party to coalesce around him. But nobody on the stage Wednesday night showed any willingness to give the former mayor any quarter.
No debate in this campaign began with more at stake - and with more expectations that it could mark an important shift in the race. Bloomberg's presence on the stage was one big reason. But so, too, was the growing reality that Sanders built up significant momentum in the first two contests and is applying it to future contests in ways that could make him ever more difficult to stop.
Ahead of the debate, a dual sensibility permeated conversation among Democratic Party strategists. One, that only Bloomberg has the resources to compete as broadly on Super Tuesday and beyond as Sanders. Two, that all the other candidates competing to become the alternative to Sanders and who have been running for the past year were not willing to concede anything to a candidate who hasn't been on any of the first four ballots.
Bloomberg supporters have not hidden their belief that only he has the potential to stop Sanders and that, in a general election, he has the resources and the positioning to defeat Trump. As the debate showed, the other candidates see him right now as an intruder of uncertain Democratic Party lineage (much like Trump with the Republicans in 2016) and resent the unprecedented amounts of his own money he has poured into the campaign.
Those ingredients are likely to combine to boost Sanders, as many Democrats predicted on the night he narrowly won New Hampshire. With Sanders increasingly consolidating the left wing of the party and the non-Sanders supporters split among others in the field, the senator from Vermont has positioned himself to gain an advantage in pledged delegates over the next 40 days of competition.
Bloomberg, the newest candidate on the stage, came in for the harshest attacks, an unruly welcome to someone who has faced no serious scrutiny since announcing his candidacy. He often struggled to parry them effectively, particularly when challenged on his record on stop-and-frisk as mayor and when called on to release women who had signed nondisclosure agreements over workplace treatment at his company.
But the attacks weren't limited to the billionaire former mayor. At one point or another, nearly everyone on the stage drew criticism from someone else. Sanders was criticized as too polarizing and for offering health-care and other proposals without a plan to pay for them.
The debate started with an explosion of attacks and rarely slowed down, with candidates vying to get into the fray and talking over one another so loudly at times that the moderators struggled to keep order. Sen. Amy Klobuchar, D-Minn., and former South Bend, Indiana, mayor Pete Buttigieg, their candidacies facing fresh challenges, sparred with one another. Former vice president Joe Biden went after Bloomberg and Sanders.
No one was more aggressive than Sen. Elizabeth Warren, D-Mass., who was nearly invisible at the New Hampshire debate two weeks ago and came to regret it. With her candidacy in jeopardy, she was relentless in attacks on Bloomberg's record.
"Democrats are not going to win if we have a nominee who has a history of hiding his tax returns, of harassing women, and of supporting racist policies like redlining and stop-and-frisk," Warren said in the opening minutes of the debate. "Look, I'll support whoever the Democratic nominee is. But understand this: Democrats take a huge risk if we just substitute one arrogant billionaire for another."
Bloomberg's presence on the stage added a new dynamic to a group of candidates who have gotten used to debating one another and often have avoided the kind of direct clashes that peppered the debate Wednesday night.
Bloomberg has focused his efforts on contests starting in March, while skipping the first four events that have reshaped the race. His spending and the results from Iowa and New Hampshire have helped him rise in national polls, a development that has triggered alarms and considerable resentment from the other candidates.
The pummeling of Bloomberg was expected, and the big question was how he would respond. At times, he fired back at his critics. At other times, he seemed to avoid jumping in when he came under fire.
Buttigieg warned Democrats that nominating either Bloomberg or Sanders would put the party at risk. "We shouldn't have to choose between one candidate who wants to burn this party down and another candidate who wants to buy this party out," he said.
On the issue of stop-and-frisk, Bloomberg repeated his pre-campaign apology for the policies he pursued as mayor. But when he tried to explain why he had sought to change the policy, Biden slammed him, claiming that it was only because of the intervention of the Obama administration and not because of Bloomberg's initiative.
During the debate, Bloomberg was attacked for operating a workplace hostile to women, for calling the Affordable Care Act a "disgrace," for failing so far to release his tax returns - he said he is working on them - and for having supported then-President George W. Bush in 2004.
Some of the most pointed exchanges, however, were between Klobuchar and Buttigieg. Their antagonism is not new, but the stakes have been raised by the tests they face in Nevada and South Carolina in proving they can appeal to a far more diverse electorate than in Iowa and New Hampshire.
The primary-season calendar is now a friend to Sanders. In past years, there has sometimes been more than a week between last of the early-state primaries and caucuses and Super Tuesday. This year, Super Tuesday follows South Carolina by three days.
Other than Sanders or Bloomberg, no candidate has sufficient money to make a real dent in the Super Tuesday states, though some have worked to put organizations into place.
The question of whether Sanders can be stopped and who might be able to do that might be getting ahead of the story, but the final moments of the debate showed how much this is at the center of the campaign.
The candidates were asked whether the person with the most pledged delegates, even if not a majority, should be rewarded with a first-ballot victory at the national convention or should the convention decide at that moment what to do. Everyone but Sanders said the convention should be allowed to work its will. Sanders stood alone on the other side.
That is what awaits the Democrats in the coming months.